1. Field of the Invention
The present invention relates to a thin film capacitor constituted with use of a high-dielectric-constant thin film, for example, a variable capacitor for voltage-controlling electrostatic capacitance by exploiting the dependence on an applied voltage of a dielectric constant in a dielectric layer formed of a high-dielectric-constant thin film, and more particularly, to a variable capacitor that operates at high frequencies with a low dielectric loss irrespective of voltage application. The invention relates also to a thin film capacitor constituted with use of a high-dielectric-constant thin film designed for use in power-source decoupling in a semiconductor integrated circuit.
2. Description of the Related Art
The higher the frequency for use in operations of wireless communication systems, electric circuits, or the like, the greater the demand for electronic components that operate at increasingly high frequencies in response to electric signals. In particular, when a thin film capacitor is used as a constituent component for a filter or resonator in a high-frequency circuit, the capacitor is required to provide a high quality factor (Q factor=1/tan δ). Moreover, in keeping with advancement and changeover to newer models in wireless communication technique, communication apparatuses adaptable to a plurality of transmission/reception systems have come to be increasingly demanded. In order to satisfy many requirements such as the demand for adaptability to a plurality of transmission/reception systems; reduction in the number of constituent components; and miniaturization, in recent years, development of variable elements such as voltage variable filters and voltage variable capacitors has been under way (for example, refer to Japanese Unexamined Patent Publication JP-A 11-260667 (1999)).
A voltage variable capacitor is a thin film capacitor in which a great dielectric constant variation resulting from voltage application to a high-dielectric-constant thin film is utilized. For example, the voltage variable capacitor is composed of a substrate; a lower electrode made of a metal thin film formed on the substrate; a high-dielectric-constant thin film formed on the lower electrode; and an upper electrode formed on the high-dielectric-constant thin film. As a material for forming the high-dielectric-constant thin film, a dielectric substance having a perovskite crystal structure has been used, for example, strontium titanate (SrTiO3); barium strontium titanate (BaxSr1−xTiyO3, hereinafter abbreviated to “BST” on occasion); and lead zirconate titanate (Pb (ZrxTi1−x)O3). It has been known that a perovskite-type oxide such as SrTiO3 or BST exhibits a high dielectric constant at about ambient temperature, and exhibits the highest dielectric constant in a composition given as: Sr/(Sr+Ba)=0.3, which is close to the boundary of cubic-tetragonal structural transformation in the perovskite crystal structure. Particularly, SrTiO3—BaTiO3 solid solution-based materials characterized by exhibiting a paraelectric phase at ambient temperature have been briskly developed, because they are ideally suited for applications in DRAM that requires high dielectric property and nonvolatile RAM that utilizes ferroelectricity. Moreover, a dielectric thin film made of such a material undergoes nonlinear dielectric-constant changes by applying a predetermined voltage thereto. In view of this, such a dielectric thin film has been regarded as suitable for variable capacitors. Further, it has been adopted for thin film capacitors designed for use in power-source decoupling in semiconductor integrated circuits that are required to meet the demands for miniaturization and large electrostatic capacitance (for example, refer to Japanese Unexamined Patent Publication JP-A 2003-209179).
The requirements to be satisfied by a variable capacitor employing such a high-dielectric-constant thin film having the above-described perovskite crystal structure are: high tunability; high Q factor; low temperature coefficient, preferably 0 ppm/° C.; high electrical strength; high insulation resistance; low distortion property; and freedom from secular change. The high tunability indicates variance in the variable capacitor. Given before-voltage-application capacitance of C0 and after-voltage-application capacitance of C1, then the tunability is expressed as: (C0−C1)/C0×100 (%), for example. Since the tunability is higher as electric field intensity is greater, it follows that, the smaller the thickness of the dielectric thin film, the higher the tunability. The Q factor is dependent upon a loss caused in each of the constituent components of the capacitor. Mainly, a dielectric loss associated with a dielectric substance and a conductor loss associated with an electrode are responsible for Q factor reduction. The capacitance and Q factor of the thin film capacitor can be obtained through impedance measurement.
The inventors of the application conducted impedance measurement on a thin film capacitor employing BST for forming a dielectric thin film. As a consequence, it has been observed that, after voltage application, the phase characteristic varies periodically with respect to frequencies (refer to FIG. 1). In FIG. 2, the measurement result is shown in terms of the Q factor. As seen from the graph, there is a tendency that a before-voltage-application Q factor (thick line) decreases monotonously, whereas an after-voltage-application Q factor (thin line) decreases periodically with respect to frequencies.
The periodic variations of the phase characteristic and Q factor with respect to frequencies are ascribable to great piezoelectricity exerted by the high-dielectric-constant thin film employed in the thin film capacitor after voltage application. That is, as the piezoelectricity becomes greater by voltage application, the dielectric thin film is excited to cause a thicknesswise longitudinal vibration by the applied high frequency, which results in appearance of a resonant characteristic based on a thicknesswise longitudinal fundamental vibration or harmonics. The resultant resonant frequency is dependent upon the thickness or configuration of a film.
In connection with such characteristics, there are two problems to be solved by the invention. The first problem is that, in fabricating a band-pass filter with use of a thin film capacitor in which the Q factor decreases monotonously with respect to frequencies as shown in FIG. 2, for example, a desired attenuation at low frequencies cannot be fulfilled without sacrificing the band-pass characteristic (Loss).
The second problem is associated with a case where a thin film capacitor employing a high-dielectric-constant thin film having the above-stated perovskite crystal structure is used as a variable capacitor.
Essentially, it is preferable that a high-dielectric-constant thin film to be employed in a variable capacitor exhibits no piezoelectricity and a range of operation temperatures thereof is Curie temperature or higher. The high-dielectric-constant thin film is preferably made of a material that exhibits paraelectricity within the range of operation temperatures. When the material exhibits ferroelectricity, it is not suited for the variable capacitor. This is because, with ferroelectricity, capacitance variations are attended with hysteresis by voltage application, and thus the capacitance is dependent upon electric-field hysteresis. Furthermore, the hysteresis corresponds to a lag behind a polarized alternating current, that is; a high dielectric loss. For these reasons, BST-based materials that exhibit paraelectricity within the range of operation temperatures (ca. ambient temperature) have hitherto been widely used for a high-dielectric-constant thin film to be employed in a variable capacitor.
The SrTiO3—BaTiO3 solid solution-based material exhibits ferroelectricity or paraelectricity according to its composition given by: Ba/Sr or Ti/(Ba+Sr). The composition is generally so determined that a paraelectric phase region may be kept within the range of operation temperatures. However, there is a possibility that the ferroelectricity of BaTiO3 appears in an abated state in a paraelectric-ferroelectric phase boundary region, which may lead to slight capacitance hysteresis.
In order to satisfy many requirements as described hereinabove, such a phase boundary region-based material may be desirable. However, what matters here is piezoelectricity derived from the ferroelectricity. The piezoelectricity is intensified by a voltage applied to vary the capacitance, which may result in occurrence of resonance in the impedance's amplitude and phase characteristics. The resultant resonant frequency is, in a variable capacitor composed of a pair of electrodes and a dielectric layer held therebetween, dependent upon each layer's film thickness and electrode size. At the periodic resonant frequency, there arise capacitance variation and Q factor reduction. The influence thereof will be problematic when it is notable within the range of usable frequencies set for the variable capacitor. That is, in the variable capacitor characterized in that the phase characteristic varies periodically by voltage application, the resultant resonance exerts an influence within the range of usable frequencies. In a case where the variable capacitor in which the Q factor is greatly reduced after voltage application is employed as a constituent component for a filter, a resonator, or the like, inconveniently, a loss that the variable element incurs increases by voltage application.
In course of the development of such a thin film capacitor in which the dependence on a voltage of the dielectric constant of the high-dielectric-constant thin film is exploited, the above-described findings and problems pertinent to voltage application have been unknown to date until the inventors of the present application had confirmed.